Digital squib driver circuit

ABSTRACT

A driving circuit for generating a required firing current for a safety device comprising an arrangement of a first transistor (M 2 ) connected in series with a second transistor (M 3 ); and a power control transistor (M 1 ) connected in series with the first transistor; characterised in that the first and second transistors operate in fully switched on mode (Rds(on)) and the required firing current (I(squib)) is generated by means of varying the voltage (Vc) across the gate source of power control transistor and the first and second transistors in a predetermined manner.

FIELD OF THE INVENTION

This invention relates to a digital squib driver architecture anddesign, in particular but not exclusively for the automotive industry.

BACKGROUND OF THE INVENTION

The automotive industry has for many years provide vehicles withelectronically controlled elements. For example, the elements may be anairbag for deployment in the event of an accident or crash. The crash isdetected typically in about 10-15 milliseconds. Then an electric signalis sent to fire the airbag. This is typically accomplished by means ofsquib which ignites a propellant (e.g. sodium azide) which generates agas (e.g. nitrogen) which in turn inflates the airbag. This stagetypically takes about 45-55 milliseconds and then within a further 75-80milliseconds the airbag deflates. A squib is a small explosive device orother firing device for a safety device or other type of device: forexample, an airbag activating component.

The energy required to fire the squib is generally provided by areservoir capacitor. This capacitor must store as much energy aspossible and as such the voltages is typically high (˜35V). An exampleof a current squib driver circuit has an output driver comprising twoswitches (a high-side and a low-side). The high-side is driven in acurrent limitation mode which generates a very high energy requirementfor the power MOSFET. For example 35V and 2.1 A for a 1 ms period wouldequate to 73.5 mJ. As the MOSFET size is related to the amount of energyto be absorbed, the higher the energy the larger the MOSFET. This canhave a large impact on the size and cost of the ASIIC.

Airbag applications also call for a safing switch in order to increasethe overall safety by adding redundancy. This is achieved by providing apower switch that is off until a redundant microchip allows the firingof the squib. The redundant chip and squib are in series. Theseguarantees that the squib does not fire the airbag until all crashalgorithms and validation are completed to confirm the need for airbagdeployment. The safing switch is generally an external MOSFET device110.

One object of the present invention is to overcome at least some of thedisadvantages of the prior art. A further object of the presentinvention is to reduce the size of components in a driving circuit.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus as described inthe accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanyingdrawings, in which:

FIG. 1 is a basic circuit diagram of a digital squib driver inaccordance with one embodiment of the present invention.

FIG. 2 is a basic circuit diagram of a two stage firming digital squibdriver in accordance with one embodiment of the present invention.

FIG. 3 is a basic circuit diagram similar to FIG. 3 showing a firstbalancing technique in accordance with one embodiment of the presentinvention.

FIG. 4 is a basic circuit diagram similar to FIG. 3 showing a secondbalancing technique in accordance with one embodiment of the presentinvention.

FIG. 5 is a basic circuit diagram similar to FIG. 3 showing a thirdbalancing technique in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 a digital squib driver 100 is shown. The squibdriver may be used to drive a squib firing in any application includingautomotive applications. The squib driver includes a safing MOSFET M1 ahi-side switch M2 and a lo-side switch M3. Both M2 and M3 are driven infull Rds (on) mode. This means that the transistor is fully turned on.The squib current I (squib) is regulated by means of the safing MOSFETM1. The Vc voltage across the gate-source of M1, M2 and M3 regulates thevoltage on Vz based on previous diagnostics, including for exampleresistance measurements of the squib (R_(squib)) high-switch (R_(HS)) orlow switch (R_(LS)). The value of I(squib) is thus equal to Vz/R total.R total is equal to the sum of R_(squib), R_(HS) and R_(LS). R_(HS) andR_(LS) are known and vary with temperature in a predictable manner whichcan be determined by measurements. R_(squib) typically varies from 1.7Ωto 4.7Ω but may be higher in certain circumstances. R_(squib) may alsobe determined by measurements.

With these resistance measurements a control circuit (not shown) cancontrol V_(C) such that the firing current is of the required valuewhilst M2 and M3 are kept fully switched on in Rds(on) mode. Thiscircuit arrangement in accordance with an embodiment of the presentinvention is much less complex and overcomes many of the disadvantagesassociated with the prior art. Notably, some of the problems of size ofMOSFETs M2 and M3 and overall circuit complex predrivers; thermalproblems associated with the switches; and energy issues are allameliorated or overcome.

In certain circumstances two stage firing is required. The circuit inFIG. 2 is an example of an embodiment which operates two or more firingsat a time. The circuit shown generally at 200 includes a safing MOSFETM1 and two firing circuits 202 and 204. Each firing circuit isequivalent to that in FIG. 1 and includes respectively high-side andlow-side switches which include MOSFETS (M2 A & B and M3 A & B). All theMOSFETS are driven in full Rds(on) mode. If the squib resistance foreach are different there will be different currents required for firingthe squib. In order to ensure that both squibs fire the value of V_(C)need to be adjusted to provide the higher of the two calculatedcurrents. This is clearly less than optimal for the squib which requiresthe lower current and for overall current consumption.

This can be rectified, at least in part by using a balancing techniquewithin each circuit 202 and 204. There are many different ways in whichthis can be achieved including:

-   -   rebalancing with a series impedance;    -   preventing simultaneous firing    -   including variable resistors in one or other circuit.

FIG. 3 shows a first balancing technique. Assuming that squib resistancedoes not substantially change during the squib life, it is possible toadd on additional resistance R_(bal1) or R_(bal2) as shown in FIG. 3.This means that the total resistance from V_(Z) to ground is roughlyequal for each squib output and thus the generated firing current ismore optimal for both squibs.

FIG. 4 offers an alternative rebalancing technique. If the squibresistance varies with time, temperature or whatever then real timerebalancing is required. This squib resistance is known in real time asdiagnostics are carried out regularly, for example every 2 ms. Based onthis measurement a logic module (not shown) can select the low sideresistance by using one or more parallel low side transistors 402. Inthis example the MOSFET is split into 3 pieces, but may be more or lessdivided as the case requires. In the example show, P1 has a resistanceof 3Ω, P2 of 6Ω and P3 of 2Ω. This results in an overall resistance of:3Ω for just P1; 2Ω for the parallel connection of P1 and P2; and 1Ω forthe parallel connection of P1, P2 and P3. This ability to vary theresistance ensures that V_(Z) for each squib circuit can be adjusted sothat firing current can be optimised, by connecting the sections orpieces of the MOSFET in parallels.

FIG. 5 shows a further improvement with respect to rebalancingparticularly where temperature impacts the circuits and resistances. Thehigh side MOSFET M2, can be split 504 in the same way as the low side502 into X parallel structures (where X is a number greater than 1). Inaddition the total resistance can be further split by inclusion of apoly-silicon resistance 506 in series with each MOSFET. In the exampleshown at 502 and 504 the poly-silicon resistance may be 0.5Ω. For 502:P1 is 1.0Ω and P3 is 1.3Ω. For 504, the parameters are such that theMOSFET has a resistance Rds(on) of about 1.0Ω maximum at roomtemperature. This is derived from 1 d 45V, wg=325 ng=50.

As has previously been noted this digital squib driver has manyadvantages over the previous analogue squib drives. Not least in respectof size, reduced requirement for sustaining energy, no regulator currentneeded for high and low side switches, simplified pre-driver circuits(not shown) can be used; improved current management especially whensquib resistance changes with time and/or temperature.

The embodiments of the present invention shown in the drawings provide anumber of advantages. These include:

-   -   Smaller scale MOSFETS are required as energy is totally        dissipated which brings about increased reliability at lower        cost.    -   The high-side switch does not need to sustain the same levels of        energy and are thus dramatically reduced in size. For example        sizes of <0.08 mm² may be achieved.    -   The high-side predriver is converted to a pure inverter driver        that does not need to regulate current. This provides faster        design time and faster test time.

The low side switch is operated in Rds(on) mode which again gives sizereduction as for the high-side. There is further no thermal couplingfrom the high-side switch. The predriver of the low side switch can alsobe simplified leading to still further size reductions.

The examples of circuit arrangements, values, devices and applicationsare shown by way of example only and may be varied whilst stillproviding the advantages of the invention.

1. A driving circuit for generating a required firing current for asafety device comprising: an arrangement of a first transistor connectedin series with a second transistor; and a power control transistorconnected in series with the first transistor; wherein, when inoperation the first and second transistors operate in fully switched onmode and the required firing current is generated by means of varyingthe voltage across the gate source of power control transistor and thefirst and second transistors in a predetermined manner.
 2. A drivingcircuit as claimed in claim 1, wherein the voltage is varied in responseto measurements of the resistance of the first and second transistorsand of the safety device.
 3. A driving circuit as claimed in claim 1,wherein the power control transistor is a safing transistor.
 4. Adriving circuit as claimed in claim 1, comprising more than onearrangements of said first and second transistors, wherein thearrangements are connected in parallel for providing one or morerequired firing current for one or more safety devices.
 5. A drivingcircuit as claimed in claim 1, further comprising a balancing elementassociated with the or each arrangement of the first and secondtransistors.
 6. A driving circuit as claimed in claim 5, wherein thebalancing element comprises a series impedance.
 7. A driving circuit asclaimed in claim 5, wherein the balancing element prevent simultaneousgeneration of the required firing current.
 8. A driving circuit asclaimed in claim 5, wherein the balancing element comprises a pluralityof resistive elements in the or each arrangement of the first and secondtransistors.
 9. A driving circuit as claimed in claim 1, wherein thesafety device includes a squib which is fired by the required firingcurrent.
 10. A driving circuit as claimed in claim 1, wherein the safetydevice includes an airbag activated by the required firing current. 11.An integrated circuit including a driving circuit as claimed in claim 1.12. A method of activating a safety device by generating a requiredfiring current using a driving circuit for comprising an arrangement ofa first transistor connected in series with a second transistor; and apower control transistor connected in series with the first transistor;the method comprises: operating the first and second transistors infully switched on mode and varying the voltage across the power controltransistor and the first and second transistors in a pre-determinedmanner in order to generate the required firing current.
 13. A drivingcircuit as claimed in claim 2, wherein the power control transistor is asafing transistor.
 14. A driving circuit as claimed in claim 2,comprising more than one arrangements of said first and secondtransistors, wherein the arrangements are connected in parallel forproviding one or more required firing current for one or more safetydevices.
 15. A driving circuit as claimed in claim 3, comprising morethan one arrangements of said first and second transistors, wherein thearrangements are connected in parallel for providing one or morerequired firing current for one or more safety devices.
 16. A drivingcircuit as claimed in claim 2, further comprising a balancing elementassociated with the or each arrangement of the first and secondtransistors.
 17. A driving circuit as claimed in claim 3, furthercomprising a balancing element associated with the or each arrangementof the first and second transistors.
 18. A driving circuit as claimed inclaim 4, further comprising a balancing element associated with the oreach arrangement of the first and second transistors.
 19. A drivingcircuit as claimed in claim 6, wherein the balancing element preventsimultaneous generation of the required firing current.
 20. A drivingcircuit as claimed in claims 19, wherein the balancing element comprisesa plurality of resistive elements in the or each arrangement of thefirst and second transistors.